This invention relates to a process for comminuting cellulose ethers.
Cellulose ethers, such as methylcellulose and hydroxypropyl methylcellulose, are used as thickeners in such diverse applications as water-based paints, cements, and foods. Cellulose ethers are prepared by reacting cellulose with alkali to produce an alkali cellulose. The alkali cellulose is reacted with an etherifying agent, such as an alkyl chloride or an epoxide, to form the cellulose ether. Cellulose ethers maintain much of the fibrous structure of the cellulose raw material. As a consequence, the cellulose ethers are generally a low density product due to the bundling of random, non-aligned fibers. The low density makes the packaging and transportation of cellulose ethers cumbersome.
In order to increase the density, most processes attempt to entangle and coalesce the cellulose ether fibers by applying shear to a hydrated cellulose ether below about 30.degree. C. This shear force applied by an attrition mill for instance, results in some densification. However, the settled densities of methylcellulose are usually still below about 0.4 g/cm.sup.3. The required increase in shear to obtain densities higher than 0.4 g/cm.sup.3 is nearly prohibitive. Also, a dusty cellulose ether product often results, because the mill breaks the cellulose ether particles into very small pieces.
The cellulose ethers are normally dried before they are milled. Unfortunately, the drying times are long due to the large diameter of the cellulose ether product.
A characteristic of the cellulose ethers made by the above process, which may not be desirable in some applications, is that the product is not dispersible in cold water. Non-dispersibility is due to the large surface area and fibrous nature of the cellulose ether. The large surface area leads to the outside particles of the cellulose ether being hydrated before the inside particles when added to water. A gelatinous membrane of hydrated outside particles is thus formed around the inside particles, shielding the inside particles from complete hydration. Thus, cellulose ethers, specifically methylcellulose and hydroxypropyl methylcellulose, form a lumpy mixture when added to cold water due to the incomplete wetting of the individual inside particles. Therefore, it is often necessary to surface-treat the cellulose ether product of these processes with, for example, a dialdehyde such as glyoxal, if dispersion in cold water is desired.
It would be desirable to have a low shear energy process for producing a free flowing cellulose ether product having a high density. If the process for producing a denser cellulose ether could also reduce the drying time and the dust associated with the milling, it would be a great benefit. Furthermore, it would be desirable to reduce the energy required in the process for milling and densification. Also, if the product particles were of a uniform size and dispersible in cold water without surface-treatment, it would be of great value.